The holes were cut so that the hackers could plug in USB drives that installed their code onto the ATMs. Details of the attacks on an unnamed European bank's cash dispensers were presented at the hacker-themed Chaos Computing Congress in Hamburg, Germany.
The thefts came to light in July after the lender involved noticed several its ATMs were being emptied. The bank discovered the criminals were vandalising the machines to use the infected USB sticks. Once the malware had been transferred, they patched the holes up. This allowed the same machines to be targeted several times without the hack being discovered.
The attackers could take the highest value banknotes in order to minimise the amount of time they were exposed. Interestingly the software required the thief to enter a second code in response to numbers shown on the ATM's screen before they could release the money and the thief could only obtain the right code by phoning another gang member and telling them the numbers displayed. This stopped the criminals going alone.